Interpretive Summary: A collaborative study was conducted involving scientists from the Livestock Issues Research Unit and Texas Tech University to evaluate the potential interactions of dietary energy concentration and source on the immune response of beef calves. Specifically, the objective of this study was to characterize the pathogenesis of bacteremia and septicemia in colostrum-fed Jersey calves and identify biochemical markers that can differentiate between bacteremia and septicemia. The results of this research will be of particular interest to beef cattle feedlot managers, veterinarians managing the health of feedlot cattle, and scientists, whether from industry, academia, or industry, working in the area of beef cattle production, health, well-being, and alternative management practices. Results of this study indicated that calves challenged with 0.0015 billion, and 0.015 billion colony-forming units of E. coli rapidly developed signs of septicemia. The degree of hypoglycemia observed was directly related to the challenge dose. Calves challenged with the intermediate dose of 150 million CFU developed a robust acute phase response with a stronger febrile response and more sustained acute phase protein synthesis. However, the acute phase response of the calves was independent of increases in peripheral concentrations of the pro-inflammatory cytokines, tumor necrosis factor-alpha or interleukin-6. The pathogenesis of E. coli bacteremia and septicemia in colostrum-fed Jersey calves are different, and within each the response is dose dependent. All Jersey calves fed colostrum were able to survive within 48 hr of the challenge. The results of this research will be of particular interest to scientists, whether from industry, academia, or industry, working in the area of bovine immunology, health, and well-being.

Technical Abstract:
Objectives of the present study were to characterize the dose dependency of an intravenous Escherichia coli (E. coli) challenge in colostrum-fed Jersey calves and to identify biochemical markers indicative of septicemia. Eighteen 3-wk old colostrum-fed Jersey calves were completely randomized to 1 of 6 doses of E. coli treatments. The challenge doses included 0, 1.5 million, 15 million, 150 million, 0.0015 billion, and 0.015 billion colony-forming units (CFU) given intravenously as a bolus in 5 mL of sterile isotonic saline. Peripheral blood samples were collected at 0, 2, 4, 8, 12, 24, and 48h relative to the challenge for blood metabolite, total leukocyte count, and differential analyses. Rectal temperatures were collected via indwelling rectal temperature probes at 5-min intervals, and hourly averages calculated from 2d prior to the challenge till 2d after the challenge. All calves survived the 48h observation period following the challenge. The behavior of calves given 0.0015 billion, and 0.015 billion CFU was altered (P < 0.01) beginning 0.5h after the challenge and returned to that of the control calves by 6 and 32h for calves challenged with 0.0015 billion, and 0.015 billion CFU, respectively. There were treatment x time interactions (P < 0.01) on total leukocyte counts and plasma glucose concentrations. Calves administered 0.0015 billion, and 0.015 billion CFU had leucopenia beginning 2h after the challenge and returning to counts similar to the control calves within 24h. Additionally, those calves were hypoglycemic from 4 to 12h after the challenge with the degree of hypoglycemia inversely related to the dose of the E. coli. There were treatment x time interactions (P < 0.001) on rectal temperatures following the challenge. All calves challenged with E. coli developed a febrile response, but the intensity and duration of the response were dependent on the challenge dose. These data indicate that calves intravenously challenged with 0.0015 billion, and 0.015 billion CFU of an E. coli showed immediate clinical and biochemical signs indicative of septicemia. However, calves administered 150 million or less of the E. coli had febrile responses, but did not develop septicemia. Blood glucose concentrations may be a dose responsive indicator that can easily be tested on-farm to differentiate between a septicemic versus non-septicemic calf.